1. Field of the Invention
The present invention relates to a laser oscillator which excites laser gas by a discharge tube to oscillate a laser beam.
2. Description of the Related Art
In general, the quality (light focusing property) of a laser beam output from a laser oscillator is improved as a mode order is lowered. In this regard, there has been known a laser oscillator configured to restrain laser oscillation of a high order mode and allow laser oscillation to occur in a low order mode. Such a laser oscillator, for example, is disclosed in Patent Registration No. 3313623, Japanese Laid-open Patent Publication No. 2013-247260, and Japanese Laid-open Patent Publication No. 2009-94161. In the laser oscillators disclosed in these Patent Literatures, an aperture is arranged between an output coupler and a rear mirror. In the laser oscillators, the diameter of laser beam is limited by the aperture, and therefore laser oscillation of a high order mode is restrained and the generation of scattered light is suppressed.
However, since the laser oscillators disclosed in the aforementioned Patent Literatures have an aperture, their configurations are complicated and the aperture absorbs a laser beam, resulting in the deterioration of laser output.
In contrast, Japanese Laid-open Patent Publication No. 2-166778 discloses a laser oscillator configured to perform coating on the surfaces of an output coupler and a rear mirror to oscillate only a laser beam of a low order mode. In this laser oscillator, a semi-transmissive film is coated on a radial center portion of the output coupler and a non-reflective film is coated at a peripheral portion thereof. However, in this laser oscillator, since the non-reflective film is coated at the peripheral portion of the output coupler, it is difficult to suppress the generation of scattered light.